Engine Cooling Tips - A Hot Topic

Of all the issues confronting the owners of custom classic trucks one of the most critical is maintaining proper engine operating temperature. Big engines, little radiators, air-conditioning condensers, and low-speed cruising can all contribute to overheating, but the major cause is often the wrong combination of components.

1/19One of the most critical decisions when building a truck is selecting the proper cooling system components. When it comes to radiators, size does matter and bigger is better. Radiators come in two basic configurations, cross-flow or down-flow-most early trucks used the latter. This down-flow example is from Speedway Motors.

Overheating an engine usually comes with consequences and the hotter the engine, the worse those consequences will be. The first sign that an engine is getting hot is usually pinging and detonation or running on after the ignition is shut off. In severe cases the abnormal combustion that takes place in a hot engine under a load can blow holes in pistons and damage rings and rod bearings. Overheating can cause pistons to scuff or even seize, exhaust valves may stick in their guides, and warped heads and blown head gaskets are common catastrophes. On the other hand, running an engine too cool has its own set of problems, among them, reduced performance and corrosive condensation collecting internally.

How Hot Is Hot?
For many old school truckers anything over 180 degrees on the temperature gauge makes beads of sweat form on their forehead, but 190 degrees is probably ideal-that's hot enough to boil off contaminants in the oil and prevent the formation of sludge. In fact, while it makes some people nervous, 200 to 210 degrees with a late-model engine isn't out of line with today's lubricants. With a 50/50 mix of ethylene glycol and water (the preferred coolant) the boiling point is raised to 220 degrees.

2/19As trucks became lower and wider, cross-flow radiators became commonplace. This one is also from Speedway Motors.

Choosing A Radiator
As a basic rule of thumb, fit the largest radiator that will fit in your truck-you'll never regret having too much radiator, the same can't be said for having too little.

Radiators can be divided into two basic types, down-flow and cross-flow. Early trucks had down-flow radiators, but as the stylists made the front sheetmetal lower and wider there wasn't room for a traditional cooler and the change was made to the cross-flow design. The reasoning behind either is easy to see, the longer path through the core exposes the coolant to more area for more effective heat transfer.

One of the great debates concerning radiators is which is better, aluminum or copper/brass. The truth is in terms of cooling the difference is minimal-appropriately sized, a radiator made from either material will work. The real differences between aluminum and copper/brass are these: Aluminum is lighter and cheaper, that's why the OEMs use it; copper/brass is easier to repair and is less susceptible to corrosion.

3/19Flex fans are made from a variety of materials including aluminum and stainless steel. Keep in mind, some serpentine drive systems reverse the direction of the water pump so a reverse rotation fan is required.

One of the newest trends in aftermarket radiator construction is the multi-pass core-in other words, the coolant passes through the core one way, then is redirected by internal baffles and flows back through the core in the opposite direction. Multiple pass radiators may use double or triple paths depending on the size of the core.

Mechanical Fans
Engine-driven fans are usually criticized for consuming a large amount of horsepower-of course if you think about it from another perspective, can a 750-watt (one horsepower) electric fan move as much air as an engine-driven fan consuming 15 hp? When an engine is under a heavy load and the A/C is on an engine-driven fan is hard to beat. Of course when the cooling demands aren't as great, horsepower is being lost unnecessarily so the trick is to determine the vehicle's needs and what fan works best the majority of the time.

Rigid fans-the King Kong of cooling fans, a six-blade rigid fan will move lots of air but take horsepower to spin and they can be noisy.

Flex fans-Simple and effective, the pitch of the blades pull lots of air at low speeds then flatten out at higher engine/vehilce speeds for less drag.

Clutch fans-there are a variety of these, the major drawback for all of them is the space they require. However, they are a good choice for an early truck if they'll fit.

Viscous-Torque limiting, just a fluid coupling that provides some drive all the time.

10/19The bent blade wreaked havoc on the core and caused the other blades to distort and the result was a ruined radiator.

Viscous-(Thermostatic) turns on and off at a predetermined temperature. The most sophisticated design, the fan "idles" and doesn't run at engine speed until needed.

The difficulty with using these is that temperature calibration is critical. According to Scott Leon, a former GM proving ground technician, production OEM clutch fans are normally calibrated to turn on when the coolant temperature reaches 220 degrees but calibrations vary. The average guy doesn't have the option of going into the parts store and asking for a 178-degree clutch, for a 16-inch fan. Even if he did, he would have to know what temperature cut-in he needed before he asked for it. Too low a cut-in and it would run all the time, too high and it will never turn on.

11/19Electric fans may be mounted in front of the radiator as a pusher or behind as a puller. Note the blades on this fan are straight.

Electric Fans
Electric fans are especially effective at low vehicle speeds-and as they aren't powered directly by the engine, horsepower isn't lost to their operation. However, big electric or multiple electric fans can draw lots of current and that current has to come from somewhere. At some point the alternator has to provide the energy and that does require horsepower to produce. Nonetheless, as electric fans don't operate continually the total amount of energy needed to operate them is probably less than a mechanical fan.

Another advantage to an electric fan is packaging-in many cases they will fit where other fans won't. For the greatest effectiveness, electric fans should have a shroud, and they work better pulling the air through the core rather than pushing it.

Thermostats
Simply stated, always use a thermostat. They ensure the engine reaches operating temperature quickly, provide the restriction necessary to build water pressure in the block, which eliminates steam pockets, and unless they malfunction, tossing them in the trash will not cure an overheating problem.

12/19Curved blade fans have become popular thanks to their lower noise level.

Two types of thermostats will be found, the poppet valve style and the sleeve style. The poppet style is either open or closed, while the sleeve style is a non-linear design and opens gradually. Sleeve style thermostats often eliminate the cycling often seen on the temperature gauge as a poppet thermostat opens and closes.

Radiator Caps
The radiator cap keeps the cooling system under pressure, generally somewhere between 12 and 15 psi. (Always follow the radiator manufacturer's recommendation when selecting a radiator cap). Pressure in the cooling system raises the boiling point of the coolant approximately 3 degress for every pound of pressure. Another benefit of a pressurized cooling system is the elimination of steam pockets in the water jackets.

Along with a spring that keeps a preset amount of pressure in the cooling system, radiator caps have a vacuum valve. When an engine is shut off, the coolant stops flowing through the radiator. However it continues to absorb heat from the heads and block. As a result the coolant temperature in the system increases as does the pressure created by the expanding liquid. The pressure often increases to the point where the radiator cap's pressure rating is exceeded and coolant is released through the radiator's overflow. When the system cools and the coolant contracts, air (or liquid) is drawn back into the radiator through the vacuum valve.

13/19Shrouds increase the effectiveness of electric fans. Note the two flaps in at the bottom of this example from Cooling Components that provide air flow at higher vehicle speeds.

Overflow Tanks
Because cooling systems do, from time to time, burp some fluids, an overflow tank is a wise addition. Allowing the cooling system to dump coolant on the ground, then suck air back in means that eventually the radiator will be low on coolant. An overflow tank catches any coolant released; it's then sucked back in when the system cools keeping it cool.